The present invention relates generally to lightning surge arresters and more particularly to an improved arc or spark gap structure for use in such arresters.
Lightning arresters are utilized to protect electrical equipment, for example, communications equipment on aircraft, from receiving damaging lightning strikes. Prior art lightning arresters include spark gap structures having variously formed electrodes which are spaced apart from one another such that high voltage and current lightning strikes arc across the spark gap and are thereby shorted to ground. Lightning strikes are thus intercepted by the lightning arrester prior to reaching the electrical equipment.
For proper operation of a lightning arrester, the arc over or break down voltage of the spark gap must remain substantially constant during the life of the arrester. A substantially constant arc over voltage permits the arrester to function properly for all voltages exceeding the rated arc over voltage, and prevents the arrester from shunting out or bypassing voltages below the rated arc over voltage. For example, as previously mentioned, aircraft communications equipment must be protected from lightning surges which would otherwise damage the equipment, yet it must also transmit and/or receive communications signals for proper operation. Received signals are of extremely low voltage levels; however, transmitted signals may reach several thousand volts and, hence, the gap must be large enough to prevent arc over of transmitted signals.
A problem encountered in many arc gap structures is that the high arc current often pits and scars the electrode surfaces and the resulting surface irregularities change the arc over voltage of the gap.
Various arc gap structures and techniques have been employed to reduce the damage which can otherwise be caused at the arc gap. For example, U.S. Pat. No. 2,906,925 discloses unequally spaced electrodes such that the arc is initiated across the minimum arc gap distance, and then moves toward the ends of the electrodes as the arc tends to lengthen. However, in this arrangement, the same areas of the electrodes serve as the initial arc over or ignition point with the result that after repeated arcs, the surfaces become pitted and scarred reducing the potential life of the lightning arrester.
Another arc gap arrangement is disclosed in U.S. Pat. No. 2,906,922, wherein two concentric electrodes form a spark gap which has a magnetic field imposed thereacross. The magnetic field causes arcs formed across the arc gap to move circularly around the concentric electrodes and thereby reduce the potential damage to the arc gap. Also, the point of arc over or arc ignition is randomly determined about the arc gap since the electrodes are equally spaced throughout the circular gap between the two electrodes. This arrangement, however, has no provision for lengthening the gap across which the arc extends to help extinguish arcs formed in the arrester and arcs are circulated over the same portion of the gap in which initial arc over or ignition occurs. Thus, while circulating arcs cause less damage than stationary arcs, any damage to the arc gap is within the ignition or initial arc over region of the gap.
It is thus apparent that the need exists for an improved arc gap structure for a lightning arrester which ensures a substantially constant arc over voltage during an extended useable lifetime.